The present invention relates to railroad equipment and in particular to improvements in the circuitry for detecting the presence of a railroad car wheel at a particular location.
There are presently available railroad wheel detectors which serve to detect the presence of a wheel at a particular location. Such detectors are used, for example, to trigger gate crossing controls, track shunts, car axle counters and various transducers such as hot box detectors, speed and acceleration measurement devices and for other similar applications.
Magnetic wheel detectors are disclosed, for example, in U.S. Pat. No. 3,151,827 and are available commercially from the Servo Corporation of America of Hicksville, New York sold under the trademarks SERVOPOLE and SERVOTRIP.
The detectors rely on variable reluctance magnetic sensing transducers which generate a voltage signal in response to a change in flux resulting from the coming and going of a railroad car wheel toward and away from the detector. Thus, the detector is usually mounted to a rail of the track. As a wheel approaches the detector, the steel wheel reacts with a magnetic circuit producing a flux change which in turn results in a generally sinusoidal output signal. The cross over point of the output signal occurs when the wheel is dead center over the sensing element.
While detectors of the type described above have operated successfully for many years, the detectors are susceptible to noise which could result in false triggering. As a result, such detectors were provided with an amplitude threshold circuit to cut out low level signals. Unfortunately, since the signal level varies with train speed, setting a threshold too high could result in missed signals from slow moving trains.
As mentioned, such detectors are rail mounted and as a result, may be subjected to extreme vibration, particularly from high speed trains. Such vibrations, by jolting the equipment, can result in short duration pulses. In order to eliminate such pulses from possible consideration as wheel signals, a time threshold is provided so that unless a given pulse exceeds a minimum time duration, it is ignored as comprising a possible wheel signal. Unfortunately, at very high speeds (in excess of 85 miles per hour) a true wheel pulse is relatively short and thus the possibility exists that such pulses may be missed for failing to exceed the time threshold set to avoid spurious signals.
From the above, it should be apparent that available wheel detectors have difficulty in detecting wheel pulses for trains moving at extremely slow (less than 5 mph) or fast (in excess of 85 mph) speeds. The former because the resultant wheel pulses may fail to exceed the amplitude threshold set to avoid noise and the latter because the resultant wheel pulses may fail to exceed the time threshold set to avoid spurious vibration signals.
Heretofore, several observations have been made regarding the output voltage signals produced by magnetic wheel detectors:
1. For a given train wheel, the zero crossing (in space), related to the center of the detector, of the output voltage signal remains substantially constant regardless of the train speed; PA1 2. For a given train wheel, the maximum amplitude of the output voltage signal varies directly with the train speed; PA1 3. As between different wheels, even on the same train, the amplitude and time duration of the output voltage signal can vary significantly even if travelling at the same speed; PA1 4. For a given speed, even as between different wheels, the distance from the output voltage signal peak to the zero crossing is substantially the same.
The above is depicted in FIGS. 1a and 1b.
In view of the above, it is the principal object of the present invention to provide an improved railroad car wheel detector particularly capable to identify, with a high degree of reliability, exceedingly fast or slow moving railroad car wheels;
A further object is to provide such a detector which utilizes conventional transducers to detect and identify passing railway car wheels;
A still further object is to provide such a detector which may be implemented at reasonable cost and which may readily be implemented into existing systems.